The whole tag line of Death Knights was that they didn't have a fixed role spec. Each tree was original built and designed with the same fluidity and intention as the Druid feral spec of the time. Play the theme (Blood/Frost/Unholy) you want; spec (Blood/Frost/Unholy) for the role you want.
Sure some builds came out of top in various niches and roles, but each spec was designed and intended to perform both roles depending the talent selection.